This invention relates generally to the field of manufacturing integrated circuits and specifically to apparatus and methods of measuring and controlling the concentration ratios for multi-fluid mixtures used in wafer processing.
In the manufacture of semiconductors, semiconductor devices are produced on thin disk-like objects called wafers. In order to produce properly functioning devices, these wafers are subjected to a number of process steps during their manufacture. For a variety of reasons, many of these process steps are performed in process tanks. For example, process tanks are used in the steps of chemical etching, photoresist stripping, cleaning/rinsing, and wafer drying to name a few. Most, if not all, of these steps require the use of one or more specific processing liquids. There is a wide variety of processing liquids, such as deionized water, RCA standard clean 1, RCA standard clean 2, ammonium hydroxide, hydrochloric acid, hydrochloric acid, or hydrogen peroxide. The exact liquid used depends on the particular step to be performed and the particular devices to be produced.
In many of these process steps, it is preferable that the processing liquids have a dissolved gas contained therein. Dissolving a gas in the processing liquid can result in a number of benefits, including: (1) improving the results sought to be achieved by that step; or (2) decreasing the time necessary to achieve the necessary results of that step. Moreover, some processing steps can not be performed effectively at all without dissolving a gas in the particular processing liquid. One example of a process step that uses a liquid containing a dissolved gas is the process of photoresist stripping using ozonated deionized water. In this step, ozone gas is dissolved into liquid deionized water. This multi-fluid mixture (i.e. the liquid deionized water containing the dissolved ozone) is then applied to wafers located in a process tank. As used in this patent, the term xe2x80x9cfluidxe2x80x9d encompasses both a liquid and a gas. As such, the term xe2x80x9cmulti-fluid mixturexe2x80x9d includes any mixture that contains at least two different chemical compounds, including liquid-gas mixtures, liquid-liquid mixtures, or gas-gas mixtures. It should be noted that the principles set forth in this patent apply to any of these xe2x80x9cmulti-fluid mixturexe2x80x9d embodiments. However, for reasons of simplicity and clarity, both the prior art and the invention will be described herein with respect to a liquid having a gas dissolved therein.
In the manufacturing steps in which a liquid containing a dissolved gas are used, it is imperative that the concentration ratio of dissolved gas to liquid remain constant at all times. This xe2x80x9cconstant concentrationxe2x80x9d requirement applies not only to the time during which a particular batch of wafers is processed, but also must be maintained from wafer batch to wafer batch. Non-constant concentration ratio can result in non-uniform etching, inconsistent stripping rates, and a host of problems that can cause devices to fail. As such, it is well established in the industry that keeping the concentration ratio of gas to liquid constant in multi-fluid mixtures is a must. However, because it is standard for a single gas or liquid reservoir to supply the necessary gas or liquid to multiple pieces of equipment simultaneously, the gas and liquid supply lines that lead into a particular piece of equipment undergoes continuous changes in pressure. Changes in pressure affect flow rates, which in turn will affect the gas and liquid concentration levels as they enter the equipment. As such, systems that maintain constant concentration levels must be employed.
Currently, constant concentration levels are maintained in multi-fluid processing mixtures by employing a separate means to control the mass flow rate on both the gas supply line and the liquid supply line. This can be done using a mass flow controller or a pressure regulator in series with a flow meter on each supply line. A simplified embodiment of one such prior art system is shown in FIG. 1. It is well known in the art that by combining a flow meter and a pressure regulator in series on a variable pressure fluid supply line, the fluid mass flow rate through that supply line can be controlled, be it a liquid or a gas. In prior art systems, a separate sub-system controller is coupled to the flow meter, and the pressure regulator of each supply line in order to facilitate mass flow control for that line. As such, in using prior art systems as illustrated in FIG. 1, a user separately establishes the mass flow rates for the liquid and the gas. The gas and liquid then independently flow into the process tank at their respectively established mass flow rates, forming a multi-fluid mixture (i.e. a liquid with a dissolved gas). It is in this way that prior art systems attempt to achieve constant concentrations of gas and liquid in the resulting multi-fluid mixture. However, this design has a number of drawbacks.
First, if for some reason either the gas mass control system or the liquid mass control system fails or becomes imprecise during wafer processing, there is no way to immediately fix the problem without disturbing the process. One must wait until the batch of wafers has gone through the entire failed process step. As such, an entire batch of wafers will be ruined. This can cost a manufacturer extraordinary amounts of money.
Second, because slight variations in the concentration ratio of the gas to liquid in the multi-fluid mixture can cause serious damage to semiconductor devices, personnel must constantly maintain the integrity of both the liquid mass control system and the separate gas mass control system. This can result in a significant usage of time, manpower, and money.
Finally, because prior art system merely set the mass flow rates of the gas and liquid, these systems can not be used to control the concentration ratio in re-circulation systems. This is because as the multi-fluid mixture is re-circulated back into the system and used again, the concentration ratio of the supplied liquid will be constantly changing, even though it is being moved through the supply line at a constant mass flow rate. As such, the concentration rate can not be controlled.
Thus, a need exists for a system and method that can automatically detect and adjust the concentration ratio of a multi-fluid mixture to ensure constant concentration ratio during a wafer processing step. This must be done without disturbing the process step.
These problems and others are solved by the present invention which in one aspect is a method for supplying a multi-fluid mixture to a process tank comprising: transporting a first fluid through a first supply line having means to control mass flow rate of the first fluid; transporting a second fluid through a second supply line having means to control mass flow rate of the second fluid; converging the first and second fluids to form a multi-fluid mixture; repetitively measuring the concentration levels of the first and second fluids in the multi-fluid mixture with a sensor; and upon the sensor detecting an undesirable concentration level in the multi-fluid mixture, the sensor automatically adjusting the mass flow rate of at least one of the first and second fluids to achieve a desired concentration level during wafer processing. The repetitive measurements of concentration levels can be many times per second, essentially continuously, or periodically according to a predetermined pattern.
Additionally, the method can further comprise the steps of filling the process tank with the multi-fluid mixture; overflowing the process tank with the multi-fluid mixture; and re-circulating the overflowed multi-fluid mixture back through the process tank, the re-circulated multi-fluid mixture being introduced back into circulation at a position downstream of the sensor.
Preferably, the first and second fluids converge to form a multi-fluid mixture prior to entering the process tank. In such an embodiment, the method will further comprise the step of transporting the multi-fluid mixture into the process tank. It is also preferable in this embodiment that the sensor be positioned to measure the concentration levels of the multi-fluid mixture before the multi-fluid mixture enters the process tank. Alternatively, the sensor can be positioned to measure the concentration levels of the multi-fluid mixture after the multi-fluid mixture enters the process tank.
Alternatively, the first and second fluids can be converged to form a multi-fluid mixture in the process tank itself.
Preferably, the first fluid is a liquid and the second fluid is a gas, wherein the sensor automatically adjusts the mass flow rate of the gas rather than the mass flow rate of the liquid. The gas can be carbon dioxide, ozone, nitrogen, chlorine, or flourine. The liquid can be deionized water, ammonium hydroxide, hydrochloric acid, hydrochloric acid, or hydrogen peroxide.
In another aspect, the invention is a system for supplying a multi-fluid mixture to a process tank comprising: a first supply line having means to control mass flow rate of a first fluid; a second supply line having means to control mass flow rate of a second fluid; wherein when the first supply line supplies the first fluid and second supply line supplies the second fluid, the first fluid and second fluid converge to form a multi-fluid mixture; a sensor for repetitively measuring the concentrations of the first and second fluids in the multi-fluid mixture; and a processor adapted to automatically adjust the mass flow rate control means of at least one of the first and second fluids when the sensor detects undesired concentration levels in the multi-fluid mixture.
It is preferable that the first fluid and second fluid converge to form the multi-fluid mixture as a result of the first and second supply lines merging into a tank inlet line prior to entering the process tank. Also preferably, the sensor is located on the tank inlet line. Alternatively, the first fluid and second fluid can converge to form the multi-fluid mixture in the process tank itself.
In regards to the system, it is preferable that the first fluid is a liquid and the second fluid is a gas, wherein the sensor is coupled to the mass flow rate control means of the gas rather than that of the liquid. The gas can be carbon dioxide, ozone, nitrogen, chlorine, or flourine. The liquid can be deionized water, ammonium hydroxide, hydrochloric acid, hydrochloric acid, or hydrogen peroxide. Finally, the means to control the mass flow rate of the first and second fluids can be mass flow controllers or a pressure regulator and a flow meter in series.